1. Field of the Invention
The present invention relates generally to nondestructive testing (NDT) of ferromagnetic ropes, cables, strands and prestressed tendons (in concrete) for flaws and fractures. The present invention relates more specifically to the nondestructive evaluation of ferromagnetic ropes, cables, strands, and prestressed tendons for flaws and fractures using magnetostrictively induced acoustic/ultrasonic waves, and the passive monitoring of crack growth and fractures through the magnetostrictive detection of acoustic emissions (AE). The present invention, in particular the detection of AE, also applies to NDT of other materials such as pipes, tubes, and plates.
2. Description of the Prior Art
The deterioration (corrosion and fracture) of individual wires which make up the main cables of structures such as suspension bridges, and the stays of cable stayed bridges, is a serious problem. Many of these bridges in the United States are well over 50 years old and the importance of addressing this problem has only increased in recent years. In order to repair and maintain these cables for bridge safety, a means for the nondestructive evaluation (NDE) of these cables for fractured wires and corrosion is urgently needed.
There currently are a number of NDE methods known in the art, whereby ultrasonic waves are used to detect the presence of breaks, fractures, corrosion, and the general deterioration of strands within a cable. Unfortunately, the ultrasonic methods described to date require some means for direct physical/acoustic contact to introduce the ultrasonic waves into the individual wires under study. Except for a few cases where the end of a cable is exposed and individual wires of the cable have sufficient exposure for transducer coupling, introducing ultrasonic waves into the individual wires in this manner is generally impractical.
The terminations, socketed areas, regions under the cable bands, and regions over the tower saddles are all generally inaccessible for existing NDE techniques such as DC magnetic field leakage and transverse impulse vibrational wave methods. These areas of cable are typically in direct contact with other structural members and, as such, cannot be readily analyzed using techniques that involve field leakage from the cable or vibrational analysis of the cable.
The stress forces associated with an ultrasonic wave traveling within a cable or metal strand change the magnetic induction of the ferromagnetic material due to the magnetostrictive effect. These changes in the magnetic induction within the cable or strand can be detected using a pick-up coil placed around or on the cable or strand.
U.S. Patent No. 3,115,774, issued to Kolb, describes a magnetostrictive drill string logging device that incorporates a vibration sensor and takes advantage of the magnetostrictive properties of the metallic drill string. Unfortunately, the accuracy of the vibration sensor utilized in the Kolb patent greatly limits the ability of the patented device to help analyze or "log" the condition of the drill string within a drill hole. While the Kolb patent discloses the use of the magnetostrictive principle to generally analyze conditions along a ferromagnetic strand, it does not disclose an apparatus and method of sufficient refinement to allow a specific analysis of the corrosion, deterioration, or fractures that might be found in something such as a ferromagnetic steel cable.
An article published in 1982 in the book "Ultrasonic Testing" edited by J. Szilard, describes the application of magnetostrictive techniques to wire rope testing for fractured strands and corrosion. However, no description of an apparatus or a method for implementation of the concept is provided in this disclosure and, in general, the article simply describes or predicts the ability to use the physical magnetostrictive principle to detect defects in wire ropes or bridge cables. The Szilard article does disclose the use of a spiraling wave that is generated magnetostrictively to detect cracks within a single rod. Such spiraling waves, however, have a very limited range over which their analysis is practical. The technique described, therefore, does not lend itself to applications using long ropes or cables.
U.S. Pat. No. 4,979,125, issued to one of the inventors of the present application, describes a nondestructive means for evaluating wire ropes and cables by using the transverse impulse vibrational wave method. This method permits the detection of flaws by recognizing certain vibrational wave amplitude and distribution patterns resulting from striking the test cable or strand with a transverse force. Tension on a test strand or cable is calculated by measuring the propagation velocity of the vibrational waves through the test object. The distribution in both amplitude and time of the waves that result allows an analysis of the existence of flaws and variations in the tension across a length of rope that may not be accessible. This transverse vibrational wave method of analysis, however, is not appropriate for many areas where the vibration of the rope is effected by external components of the bridge structure or other external forces on the free movement of the wire strands.
It would, therefore, be advantageous to develop an NDE method useful for testing ferromagnetic wires, ropes, and wire strands and the like which would not require a direct access to the material under test. It would be advantageous if such a remote system could detect minor, as well as major rope flaws, stresses, and corrosions.
One method for monitoring the fracturing of wires in a steel cable or strand is by acoustic emission (AE) detection using a piezoelectric sensor. This requires a precise acousting coupling with the strand through direct physical contact between the sensor and the strand. It also typically involves a certain amount of surface preparation such as the removal of paint and corrosion in the immediate area. The durability of such sensors, when maintained in direct contact with the cable strands, very low. It is also quite complicated to install such piezoelectric sensors and careful analysis of the area of coupling is required to eliminated the effects of immediate structural discrepancies. The systems are also typically quite expensive, not only in apparatus costs, but in installation costs as well.
It is also known to use ultrasonic detectors to passively monitor acousting emissions generated by progressing fractures within a cable or metallic strand. Typically, such applications utilize piezoelectric sensors that must be physically and acoustically coupled to the material under analysis. It is also apparent that frequency sensitivity limits the range and accuracy of such methods, even if the coupling requirements are met.
a. Background on the Magnetostrictive Effect
The magnetostrictive effect is a property that is peculiar to ferromagnetic materials. The magnetostrictive effect refers to the phenomena of physical dimensional change with variations in magnetization. The effect is widely used to make vibrating elements for sonar transducers, hydrophones, and magnetostrictive delay lines for electric signals. It has also been used to excite spiraling waves in a steel rod or wire of diameters up to 14 mm for nondestructive inspection during the process of drawings the steel rod. This approach for using the magnetostrictive effect with steel rods upon drawing is similar to the non-contact generation and detection of ultrasonic waves in non-ferrous materials using electromagnetic acoustic transducers that rely on the Lorentz force.
In magnetostrictive delay line applications, the generation and detection of ultrasonic waves is typically achieved by the introduction of a sinusoidal or pulsed current into a transmitting coil surrounding a magnetostrictive delay line which is typically no more than 1 in diameter. The change of magnetization within the rod or line located within the transmitting coil causes the material to change its length locally in a direction parallel to the applied field. This abrupt local dimensional change, which is the magnetostrictive effect, generates a stress wave that simultaneously travels at the speed of sound within the material towards both ends of the rod. When the stress wave is reflected back from the end of the rod, or from a defect in the rod and reaches a receiving coil, which is in many respects similar to the transmitting coil, it generates a changing magnetic flux in the receiving coil as a result of the inverse magnetostrictive effect. This changing magnetic flux in turn induces an electric voltage within the receiving coil that is proportional to the magnitude of the stress wave. The magnetostrictive effect is independent of the direction of the applied magnetic field, so an equivalent stress is generated during each half cycle of the applied magnetic field and the frequency of the stress wave is twice that of the current applied to the transmitting coil. The frequency of the stress wave can be made the same as that of the current by applying a bias magnetic field to the rod using a permanent magnet or an electromagnet.
b. Comparison of Rope/Cable NDE Methods
The present invention utilizes the ability of the magnetostrictive effect to induce and detect acoustic/ultrasonic waves in ferromagnetic material without direct physical contact with the material. This approach overcomes the difficulties in applying conventional ultrasonic techniques to NDE of bridge cables and strands. Table 1 provides a summary comparison of techniques, including the present inventions magnetostrictive acoustics/ultrasonics approach, as applied to wire ropes and cables. The present invention also provides a number of advantages over conventional methods when applied to AE monitoring as summarized in Table 2.
TABLE 1 ______________________________________ COMPARISON OF ROPE/CABLE NDE METHODS Transverse Magnetostrict- Impulse ive Acoustics/ Magnetic Flux Vibrational Ultrasonics Leakage Wave (MsAU) ______________________________________ Principle Detection of Detection of Detection of magnetic mechanical acoustic/ultra leakage field vibrational sonic signals caused by signals reflected from wire breaks reflected defects from defects Applicable Ferromagnetic Metals, Ferromagnetic, Material nonmetals extendable to non- ferromagnetic Sensors Electro- or Mechanical MsAU permanent impulse transmitter magnet, generator and receiver magnetic- (e.g., field sensor hammer), (e.g., Hall displacement probe) sensor Couplant Not required Not required Not required Inspection About the Hundreds of Hundreds of Range size of feet or more feet or more sensor Scanning Required Not required Not required Detectable Variable 10% or 4% or less Defect Size higher cross- cross- sectional area sectional change area change Inspection No No Yes of Obstructed Areas (such as sockets, saddles, bands) Portability Poor Good Good of Equipment Simplicity Moderate Moderate to High of Data high Docu- menta- tion and Analysis ______________________________________
TABLE 2 ______________________________________ COMPARISON OF MAGNETOSTRICTIVE ACOUSTIC EMISSION (MsAE) AND CONVENTIONAL ACOUSTIC EMISSION (AE) METHODS MsAE Conventional AE ______________________________________ Sensor Magnetostrictive Piezoelectric Couplant Not required Required Physical Contact Not required Required to the Part Surface Not required Required Preparation (removal of paint, etc.) Sensor Durability High Low (under physical abuse and weather) Sensor Simple Complicated Installation Cost Low High ______________________________________